This invention relates to electro-hydraulic clutches and more particularly to a method and apparatus for compensating for hysteresis in such clutches to improve their inching characteristics.
In a typical tractor using electro-hydraulic clutches, movement of a clutch pedal controls generation of a signal that is applied to the solenoid of a solenoid-actuated valve, the valve in turn controlling hydraulic pressure in a clutch to vary torque transmission through the clutch from an engine to the wheels. The clutches exhibit hysteresis primarily due to the solenoid valve. That is, the response of a clutch to a given magnitude signal applied to the solenoid is different depending on whether the signal is increasing or decreasing. Although undesirable, hysteresis is not a problem when the rate of signal change versus clutch pedal position is high. However, it is common to use a very low rate of signal change vs. clutch pedal position in the lower portion of the range of clutch pedal travel to provide fine control for maneuvering the tractor. Because of this, any hysteresis in the system makes it difficult to xe2x80x9cinchxe2x80x9d the tractor. To illustrate the problem, assume a clutch pedal has been released to a point where any further release will result in tractor movement. If the pedal is then further released a small distance the tractor begins to move. To stop the tractor, the clutch pedal must be depressed a greater distance than the small distance which initiated movement.
An object of the present invention is to provide a method and apparatus for compensating for hysteresis in electro-hydraulic clutches.
Another object of the invention is to provide a method of compensating for hysteresis in an electro-hydraulic clutch having a solenoid controlling a valve that in turn controls hydraulic pressure in the clutch, the method comprising: sensing the position of a clutch pedal and generating, from the sensed clutch pedal position a value defining the magnitude of a signal to be applied to said solenoid; establishing a window having a top limit and bottom limit defining a range of positions of a clutch pedal; moving the window so that the top limit coincides with the sensed clutch pedal position when the clutch pedal is being released; and modifying said value when a sensed position of the clutch pedal is not greater than the top limit.
A further object of the invention is to provide an apparatus for carrying out a method as described above.
According to the method of the invention, the gain in the signal applied to a clutch solenoid is increased when the direction of clutch pedal movement changes. This is accomplished by establishing an anti-hysteresis window representing a range of clutch pedal positions. When the clutch pedal is operating within the window, the gain is relatively high. If the clutch pedal moves outside the window, the window is dragged along with the pedal so that the pedal will be operating within the window as soon as the direction of pedal travel changes.